Supervisory signaling in telephone systems



- a.. A. MILLE` ETAL SUPERVISORY SIGNALING IN TELEPHONE SYSTEMS Filed Dec. 27, 1946 3 Sheets-Sheet l Bf TQQQM,

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SUPERVISORY SIGNALING IN TELEPHONE SYSTEMS Filed Dec. 27, 1946 3 Sheets-Sheet 2 l' 1R fe a l 'I (L K ,le a E] mi. OFF/CE N0. 2

i LAK/LE /NVENTORS- ATTORNEV SeptI 13, 1949. l.. A. ILLE Em. 2,481,932

SUPERVISORY SIGNALING IN TELEPHON SYSTEMS Filed Dec. 2v. 194e V:s sheets-sheet s @V t m 3 /FFEREA/ TOLL OFF/CE IVO. 2

ATTORNEY Patented Sept. 13, 1949 NITED STATES PATENT OFFICE SUPERVISORY SIGNALING IN TELEPHONE SYSTEMS Application December 27, 1946, Serial No. 718,780

3 Claims. 1

This invention relates to telephone systems and particularly to systems including automatic switches for completing connections between calling and called subscriber and operator stations.

Objects of the invention are the provision of means for facilitating the completion of delayed toll calls in areas in which the toll board is decentralized, the provision of means for giving an inward toll operator access to a plurality of delayed-call operators, and in general the provision of means for decentralizing toll switchboards.

This invention is a telephone system in which an outgoing trunk circuit in one toll office and an incoming trunk circuit at a delayed-call operator position in another toll office are arranged to give the operator in the first toll office answering supervision responsive to the reversal of the current in a loop circuit over the interconnecting intertoll trunk, in which rering of the delayedcall operator is effected by operation of a differential relay in the incoming trunk circuit responsive to the opening of the loop circuit and connection of ground to a single conductor of the intertoll trunk, and in which ring-back of the l operator in the first toll office is effected by the release of a marginal relay in the outgoing trunk circuit responsive to a reduction of the voltage applied to the intertoll trunk in the incoming trunk circuit associated with the delayed-call operator position in the other toll office.

The invention and its features will be further explained by describing in detail a telephone system in which the invention is embodied, such a system being represented schematically in the drawing which forms a part of this specification and consists of three figures.

Referring to the drawing:

Fig. 1 shows a local office and a first toll office, the local office being represented by the subscriber station A, line LI and a switching trunk STI. The toll office is represented by a cord CI, jacks Jl and J 2, operator telephone set 8, position circuit I Il, key-set circuit 9, outgoing trunk circuit OTl, link circuit 43 and sender circuit 44;

Figs. 2 and 3 show a second toll office comprising an incoming selector IS, an incoming trunk circuit ITI connected to jack J3, a cord circuit C2, a toll operator position circuit 99, and a switching trunk ST2 connected to jack J 4. Fig. 3 further shows a local office terminating the switching trunk ST2, a subscriber line L2 and subscriber station B.

The subscriber stations A and B may be of the usual common battery type and the connections between subscriber lines LI and L2 and switching trunks STI and ST2 may be established either by manually operated or dial controlled switching means. Reference may be had to the patent to Carpenter and Hersey 1,780,906, November 11,

2 1930 and to the patent to Henry 1,916,760, July 4, 1933 for disclosure of cord, position and key-set circuits arranged for key pulsing and the associated trunk, link and sender circuits. The switching trunks STI and ST2, cord circuits CI and C2, position circuits III and 99 and the associated operator telephone and key-set circuits may be similar to those disclosed in the Carpenter-Hersey and Henry patents. The link circuit 43 and sender circuit 44 are similar to link and sender circuits disclosed in the aforementioned Henry patent. The position circuit 99 in the second toll office includes similar operators telephone and key-set circuits.

The incoming selector IS comprises a switch of the Well-known two-motion, step-by-step type arranged to give access to 16 delayed-call operator positions. A four-conductor terminal bank is provided and the selector has two sleeve brushes, thereby permitting the ten sets of terminals in each level to be divided into two groups of five sets each. To enable the selection of trunks in a desired one of the two groups on a selected level, relay E'I is provided for transferring the trunk testing circuit from one sleeve brush to the other. In one group, the terminals which are engageable by the normally connected one of the sleeve brushes are grounded; and in the other group the terminals which are engageable by the other sleeve brush are grounded. A set of level springs 66 are provided for operation on particular levels. If the brushes are stepped up to the fifth level, only the right contacts of springs 66 are actuated to effect the release of the brushes to normal; and if the brushes are stepped up to the tenth level, all of the contacts of springs 66 are actuated to effect the return of the brushes to normal and also to effect the operation of brush switching relay 61. When operated, relay 61 locks and causes the brushes to be restored to normal. While only two-digit numbers are required for selecting any desired one of these operators, the controlling sender in the first toll ofce may be arranged to transmit a larger number of digits for instance 4. The delayed-call operators trunk groups will, in such a case be numbered NNNX and NNOX where N is a particular digit, for instance 5, O is the digit used for operating relay 61', and X is any digit other than 5 or O. Thus, the levels corresponding to these particular two digits, O and 5, cannot be used for giving access to delayed-call operators and only eight levels of the switch are available for trunks to different delayed-call operators, two groups of trunks per level.

Assume now that the subscriber at station B initiates a toll call to station A and that at the time the call is initiated it is not possible for the call to be completed immediately, either because the called station is busy or because the call is not answered. In such a case, the calling subscriber is requested to replace the receiver and the call is referred to a delayed-call roperator in the calling toll oice, and the number of this delayed-coil operator is 'given to the toll operator in the called toll office.

As soon as the toll operator in the Called toll oice is successful in extending al connection, through a cord CI and a switching; trunk STE,

to the called subscriber station and the call is.

answered thereat, the calling lplug of cord Cl is inserted in jack J 2 of an outgoing trunk circuit OTI associated with an idle trunk to toll oice No. 2. A circuit is closed vfrom battery (not shown) in the `position circuit l0, through the sleeve conductors ofY the calling plug and jack J 2, conductor lf3, and the high resistance-right windingof relay 2B, to ground. Relay 26v operates, closing a circuit for operating relay 351. lllhe operation of relay 35 connects ground-through its inner left front contact, continuity back contact lof relay 38 and the outerv right back contact ofl relay 3| to start conductor 45 leading to the link circuit 43, thereby causing the link to hunt vfor and connect with thetrunk circuit OTIv and causing the link to hunt for and connect withan idle sender circuit 44'. The operation of relay 35 closes a connection from battery through the winding -of relay 38, inner right back contact `of relay 3|, andl front contact of relay 35, tov conductor 4| leading to the link circuit 43 and closes la connection from groundl through the normally closed contact of relay 29, a back contact i relay 30, the outer right front contact of relay 35v and the left winding of relay 25- to conductor I3. The left winding of relay-21B is a lowy resistance windingl so that the current through conductor |31 is increased sufficiently to cause the operation of a marginal relay in the position circuitl I5 and the lighting of a supervisory lamp (not shown) associated with the calling plug of cord CI. the link `circuit 43 connects Withl the trunk circuit O'I'l,l ground (not shown) in the link circuit, is connected to conductor 4| .to operate relay 38 in the trunk circuit. The oper-ationofrelay 38 disconnects ground from the st-art conductor 40; and relay 33 locks through itsv outer right -front contact to the` operating ground independent of relay 3|. The operation ofA relay- 38 also closes a circuit for operating relay 3|. The operation of relay 3| closes a lockingyCircuit Ithroughits inner left'front contact, to ground at a front contact ofV relay- 35. Relay 3|v opens theA operating circuit for relay 38; prepares a circuit for operating` relay 25 when relay 38 releases; connects the lower conductor of trunk 55, to the lower left winding of repeating coil 251; and closes a circuit for 'operating relay 2l', this circuit being traced from the winding of relay 2l thro-ugh the inner right front Icontact of relay 38, inner right f-rontcontact ofl relay 3|, to ground at a front contact of relay 35. When the link circuit 43 finds and. connects with an idle sender 44, battery is connected through a 10,000-ohm resistance (not shown) to conductor 421, operating relay 33v When' conductors: 45, 46', 41 and 48, a Signal is transmitted from sender 44, through. conductors 45 and 46, back contacts of relay of outgoing trunk circuit OTI, back contacts of relay I4, conductors and.- |2, through the tip and ring conductors of jack J2 and plug of cord Cl to the keyset circuit 9, thereby to effect the lighting of `a sender lamp in the key circuit as an indication that the keying of the -delayed-call operators number may be started. At the same time a loop connection is closed in the sender through conductors 4l and 48 and back contacts of relay 25, conductors of trunk 50, back contacts of relays 54 `and 56 of selector IS in toll oiiice No. 2 and the windings of line relay 5| of this selector. Relay 5| operates, closing a circuit for operating slow-to-release relay 52. The further operation of selectorl I-S occurs when a train of impulsesv is received over trunk 50 ashereinafter described.

If an abnormal condition occurs in the sender- -44 while the link and sender are attached tothe trunk circuit OTI, ground isintermittently connectecl to conductor 42 in placev of a battery through 10,000 ohms resistance. Each connec-A tion of ground to conductor 42. causes the releasecf relay 331, thereby closing a, circuit fiorl operaia. ing rel-ay 23, this circuit being traced from-groundthrough theinner left front Contact off relay 35, inner left front contact off relay, 381, back contact of relay 33, outer left front contact of relayI 35, right liront contact of relay 32', lowerwinding of relay 29, a front contact of relay 35, and` through` resistor 35 to battery; Each time that relay 29f operates, ground is disconnected. from the lette hand winding of relay 26', thereby increasing-the resistance in the circuit throughconductor |3=v and the-sleeve conductor of cord CI to extinguishl the calling cord supervisory lamp. This lamp is thus flashed until the cord is= disconnected; fromA jack J2; 4whereupon relays 26, 35, 38, 3| andv 32v release, restoring the outgoing trunk circuit to normal.

Assume-that the operator completesthe keying.v of the delayed-calli operators number andi that the start key inl the key circuit is depressed to effect the transmissionI of' a start signals tol thesender inthe mannerl describeclf in. the aforementioned Henry patent, toindicate that` keying has been completed; The sender now.y transmits theJ digits registered' therein, which digits are the digits. of the number of' the delayed-call operator in toll oi'ice No. 2 having the` ticket offV the delayed call from station B: tol station A. These impulsesare transmitted through the link circuit 43 and conductors 421i and 48, back contacts ci relay 25, over the conductors. of. trunk 59 to operate incoming selector IS as: hereinafter described. When all of the digits have been transmitted, the sender is released and'y the link returns to normal', inthe manner described in the aforementioned Henry patent, thereby disconnecting ground. from conductor 4`|1 to release relay 38 in the outgoing trunk circuit. Thel release or relay 33-completes a circuit, for operatingrelayV f 25, from the winding of relay 25 through the continuity: back contact of relay 38, right inner front Contact oi relay 3| to ground at: a front contact of relay 35. The release of relay 33 also opens theY operating circuit of relay 2-7'. The operation of relay 25' opens the connection between cord Cl'- and sender-44;' connects the left windings of repeating coil 25 through its outer front contacts, back contacts of relay i'yand conductors andl |2, to thevtalking conductorsV of cord CI; disconnects the conductors 4T and 43 from the conductors of trunk 50 and closes a supervisory bridge across the conductors of trunk 50. This bridge is traced from the lower conductor of trunk 55, outer left front contact of relay 3l, lower right winding of repeating coil 25, upper winding of polarized supervisory relay 2Q, a back contact of relay 30, winding of marginal supervisory relay 23, upper right winding of repeating coil and through a front contact of relay to the upper conductor of trunk 56.

Returning now to the transmission of the trains of impulses corresponding to the four digits of the delayed-call operators number, each impulse is transmitted from the sender by opening the loop across the conductors of trunk 5i). The line relay 5l of the incoming selector releases responsive to each impulse, but relay 52 is slow to release and remains operated during the receipt of digit impulses. Each release of relay 5i closes a circuit from ground through the inner lower back contact of relay 54, upper back contact of relay 5l, upper front contact of relay 52, winding of relay 53, upper normally closed contact of the eleventhrotary-step springs 65, through the winding of vertical stepping magnet SI to battery. Stepping magnet 6I and relay 53 are thereby energized, the operation of stepping magnet El being effective to step the brushes 1l, l2, i3 and 'I4 up to the rst level of the bank. Each time that relay 5i releases and reoperates, the circuit through the Winding of vertical stepping magnet 6l and relay 53 is closed and opened, thereby causing the operation and release of stepping magnet 5l. Being slow to release, relay 53 remains operated until relay 5I reoperates at the end of each train o impulses. The brushes of selector IS are thus stepped up to the level corresponding to the first train of impulses transmitted by the sender 44. The vertical oli-normal springs 54 are actuated as soon as the shaft of the switch moves out of normal position closing a circuit from battery through the winding of relay 55, lower contact or" springs 64, front contact of relay 55, to ground at the back contact of the release magnet 53. Relay 55 is thereby operated; and it locks independent of relay 53, the locking circuit being traced from the winding of relay 55 through the lower contact of springs 54, back contact of rotary stepping magnet 52, outer front contact oi relay 55, to ground at the inner lower front contact of relay 52,. Since the rst two digits of the delayed-call operators number are to be absorbed, the brushes are advanced to the fifth level responsive to each of these digits thereby actuating only the right-hand contacts of level springs 55. When relay 53 releases at the end of each of these digits, a circuit is closed from battery through the winding oi release magnet 53, upper front contact of vertical off-normal springs 54, right front contact of level springs 55, inner front `Contact of relay 55, back contact of relay 53, to ground at the continuity back contact of release magnet 63. The operation of release magnet 53 causes the return of the shaft and brushes to normal position. The release magnet locks under control of springs 54. When the switch shaft reaches normal, springs 64 return to normal and the release magnet 63 is deenergized. If the third digit transmitted over trunk 50` from the sender 44 is the digit 5, this digit is absorbed in like manner. If the third digit received over trunk 55 from sender 44 is the digit O, the actuation of the left contacts of level springs 55 closes a circuit for operating relay 51; and relay 6l' locks to ground at the lower iront contact of relay 52.

With relay 51 operated, the test circuit of the selector is transferred from test brush 14 to test brush 13. As soon as relay 53 releases at the end of the train of impulses, release magnet 63 is operated thereby causing the brushes to be again restored to normal. When the fourth digit is transmitted by the sender over the conductors of trunk 50, relay 5| controls the operation of stepping magnet 6| to advance the brushes of the selector to the particular level in which trunk hunting is to take place. When relay 53 releases at the end of the fourth train of impulses incoming to selector IS, a circuit is closed from battery through the winding of rotary stepping magnet E2. back contact of the level spring 66, inner front Contact of relay 55, back contact of relay 53 to ground at the continuity back contact of release magnet S3. The operation of the rotary stepping magnet advances the brushes into engagement with the first set of terminals in the selected level and opens the locking circuit of relay 55 causing the release of this relay. The release of relay 55 causes the release of the rotary stepping magnet 62, thereby closing the busy testing circuit which includes brush 14 if relay 51 is not operated and includes brush 'I3 if relay 61 is operated. If the first lset of terminals is in the desired trunk group but the associated trunk is busy or if this set of terminals is not connected to a trunk in the desired group, the terminal engaged by the connected one of test brushes 13 and i4 will encounter ground potential, and this ground is connected through the test brush and contact of relay 67, through the inner upper back contact of relay 54, lower normally closed contact of the eleventhrotary-step springs 55, back contact of the rotary stepping magnet 62, lower contact of the verticaloiT-normal springs 54, and through the winding of relay 55 to battery. Relay 55 is reoperated in this circuit thereby reclosing the circuit for operating the rotary stepping magnet 62. The reoperation of stepping magnet 62 opens the operating circuit of relay 55 and steps the brushes of the switch to the next set of terminals. When the connected one of test brushes 13 and 14, encounters a test terminal which is not connected to ground, relay 54 is operatively energized in a circuit traced from battery through the winding of relay 55, lower contact of vertical-olT-normal springs 5 4, back contact of rotary stepping magnet E2, lower contact of the eleventh-rotary-step springs 65, winding of relay 54, to ground at a front contact of relay 52. Being marginal, relay 55 does not operate in series with relay 54, so that no further stepping of the switch occurs. Relay 54 closes a connection from ground through the lower front contact of relay 52, inner upper front contact of relay 54, contact of relay 6l and test brush I3 or 14, to the test terminal of the trunk thereby guarding the trunk against seizure of any other hunting selector. The operation of relay 54 disconnects the conductors of trunk 55 from the windings of relay 5l and extends these conductors through brushes l! and 'l2 to the tip and ring conductors of the trunk which is connected to the selected set of terminals. The release of relay 5| causes the release of relay 52. Relay 54 is held operated, after relay 52 releases, by ground supplied over the sleeve conductor of the selected trunk circuit, as hereinafter described.

Assume n-ow that the selected terminals in the bank of switch IS are connected to the trunk ITI, shown in Fig. 3. When relay 54 operates, a circuit is closed for operating relay 8l] of the incoming trunk circuit ITI and relay 23 of the outgoing trunk circuit OTII. This circuit isl traced from battery through the lower' winding or relay 9F31, a. ba-ck contact of relayy SII', lower winding of relay 82, lower left winding ot repeating coil:v 95, conductor '18, brush 12 a front conta-ct ofi relay B, lower conductor of trunk-4 50, a front contact of relay 31, lower rightwinding. of repeating; coil upper winding of polarized supervisory relay 2li', a back contact of relay 39, winding of marginal supervisory relay 23;. upper right winding. or reh peating coil 29, a iront Contact of relay 25, upper conductor of trunk 59,. upper front contact of relay 5'4, brush 1I, conductor 'II, upper left wind-- ing of repeating coil 85,. upper winding of relay 8-2, a back contact of rel-ay 8|, and. through the upper winding of relayy Sily to' ground, Relay 92 does not operate at this timev since its windings are4 dierentiall-y energized andv relay' 29 does not operate because the current. through its: uriner winding is in thev non-operate direction. rEhe operation of relay 89 closes a circuit for operating the slow-to-release rela-y 9G' and the opera* tion of relay 23 closes a circuit for operating relay 21. Relay 99 connects' ground toA conductor t9 tof maintain the energization of relay 54 of incoming selector IS; closes a circuit for operating relay 92, and closes a circuit for lighting the answering lamp'189 associated with jack J3. The operation of relay 92 connects ground toconductor "I9, and disconnects; groundfrom the low resistance lett-hand Winding of relay- 931 Relay S9 is slow-tosre-l'ease in order to maintain the operation of relayv 54 of the incoming selectoi1 in caserelayV SIBV isV momentarily released due toa ringing key operation or some other disturbance over the trunlg W'hen the called delayed-call operator inserts a plug ot a' cord C2 inl jack J3 to answer the call, battery in the position circuit S9- isy connected through thesleeve conductor of the plug an'djack tooperatively energize the right winding. of relay 93; The operation of relay 93- closes a circuit for operating relay 8|. The. operationV of relay Il! closes a circuit for operating relayA 9|- and` interchanges they connections-between thewindings of relay 89 and conductors 'I'I and, 78, thereby reversing the current over trunk 59 to causev the operation of relay 29 of outgoing trunk circuit OTI. Relay 23v releases momentarily when the current over trunk 50, isreversed but not long enough-to releaserelay 21'. Thesoperation' or relay 29 disconnectsA ground' from the low resistance winding of relay 26, thereby to eiect the extin.- guishing oi the cordy supervisory lamp in.. usual andl well-known mannner'; Relay 9-I ci the incoming trunk circuit locks'under control: of relay 92A', andV opens the' circuit for lighting lamp' 89'. The delayed-call operator is now informed that the toll operator in toll oce No. 1' has a connec tion established with ther called station A. The delayed-call operator' thereuponr extendsl a' connection over switching trunk ST2,through' the local oice andVl line L2^to the calling station B.

Ii the operator in toll ofh'ce No; 1 wishesto sig'- nal the delayed-call operator after the delayed call operator has restored'the talking key of cord C2 to normal, the ringing key1 of cord` CI is operated thereby connecting battery o'fa high: enough voltage to the tip conductor` of` the calling plug to effect. the operation orA relay' 23 of outgoing trunk circuit OTI. The operation of relay' 28 closes a circuit, including the front: contact of relay 29, for operating relay' 39. Relay 301 locks lunder'control of relay' 2.8:butzindependent of'relay 29, closesf a circuiti for: holding relayl 275 operated to prevent false" ring-back, opens thel connection between the left winding of relay 26 and' the: back contact of relay 29 to preventy false supervisionto the toll operator in toll office No. l, opens the supervisory bridge across trunk 59 and connects ground to and through the winding of relay 23, upper right winding of repeating coil 29; front contact of relay 25, upper conductor of trunk 5.0,. a front contact of relay 54' of selector IS, brush 1I, conductor TI, upper left winding of repeating coil 8'5 of incoming trunk circuit ITI in toll'. oirlce No. 2', upper Winding of relay 82, inner lower front contact of relay E'IV and through the lower wind'- ing of relay to battery. Relay 23 of trunk circuit O'II may or may not remain operated in this circuit, relay 89 of incoming trunk circuit ITI is held operated by its lower winding and relay 82 is operated' by the energization of its upperI wind'- ing alone. The operation of relay 82 closes. a'- con'- nection from ground through its front contact, winding of relay 94, continuity back contact of relay 96, and sleeve conductor of jack J3' and cord C2 to battery' in the cordI circuit, thereby eiectng the operation of relay 94. The resistance of the winding of relay 94 is approximately the same as that of the low resistance Winding of relay 93 so as to cause the lighting of the supervisory lamp associated with the plugi con.- nected to jack J3. When the' ringing keyof cord CI is released relays 28 and 3e' release and so: also does relay 82; but relay 94' islocked independent of relay 82 and connects the winding. of relay 95 to interrupter 95. Relay 96' is alternately operiated' and released under the control of interrupter 95 to intermittently open the operating circuit of relay 94' through the sleeve conductor. Each time relay 96 operates, the cord sup'ervisory lamp) is extinguished' but relay 94' is held operated in a circuit from ground through its own left front contact and winding, front contact ofv relay 96, resistor 97, and right front contact oi' relay 99 to battery. The cord supervisory lamp isy thus flashed to recall the delayed-call operator. When the talking key of cord C2' is again' operated, the resistance in the sleeve conductor circuit is inicreased to cause the release of relay 94; and the release of relay 94 disconnects the winding of relay 96 from the interrupter.

If' the delayed-call operator wishesv toy signal the inward toll' operator in toll oiiice No. I, operation of the ringing key of' cord' C2 connects battery of high enough voltage to the tip conductor of the cord to cause the operation of relay' 81. With relay 8'I operated ground is connected through resistor 83 and the-front contact of relay 81 to the lower'windi'ng ofrelay 8Uv thereby hold'- ing relay 8B' operated; and this ground is further connected through the inner lower front contact of relay 8| to the upper winding of relay' 82, whereby the voltage applied' through thisl wind-'- ing to the loop circuit over trunk 50 is reduced and the current thereover is insufficient tomaintain the operation of marginal supervisory relay 23 of outgoing trunk circuit OTI. Relay 29? remains operated since the current is maintained in the operate direction. The release of relay 23 causes the release of relay 2l thereby closing a circuitk from ground through the left contact of relay 29', back contact of relay 27 and winding of relay I4'. Relay' I4 is operated connecting bat'- tery through resistorv I5 and conductor II to the tip conductor'of cord' CI'. If the connection` with station A includes another toll'orlice,l this battery operates a relay in the intertoll trunk' circuit associated with the other end' of cord-Y C Ii toE ei'e'ct the transmission of a ring-back signal to the toll operator in the other toll cnice. When the delayed-call operator in toll oiiice No. 2 releases the ringing key of cord C2, relay 81 of incoming trunk circuit ITI releases, relay 23 of outgoing trunk circuit OTI reoperates, and relays 2l and I4 release thereby terminating the transmission of the ring-back signal.

When the delayed-call operator disconnects cord C2 from jack J3, relay 93 releases. Relays 80, 9U, 9I and 92 remain operated until trunk 50 is released in toll oice No. 1. The release of relay 93 while relay 92 is operated causes the release of relay BI thereby reversing the current over trunk 59 to cause the release of relay 29 of outgoing trunk circuit OTI. The release of relay 29 connects ground to the low resistance Winding of relay 2S to light the cord supervisory lamp. When the inward tool operator in toll ofIice No. 1 disconnects cord CI from jack J2, relay 26 releases causing the release of relays 35, 3| and 25. The release of relays 3| and 25 opens the supervisory bridge across trunk 50 causing the release of relay 89 of incoming trunk circuit ITI. The release of relay 89 causes the release of relays 99, 92 and 9i. If the cord C2 has not yet been disconnected from jack J 3 when relay 92 releases, the cord supervisory lamp is lighted as a disconnect signal. With cords CI and C2 both disconnected from jacks J2 and J3, trunk circuits OTI and ITI are restored to normal. The aforementioned release of relays 9D and 92 of trunk circuit ITI disconnects ground from conductor 'I9 thereby causing the release of. relay 54 of selector IS. The release of relay 54 closes the circuit for operating release magnet 63 and the shaft and brushes of selector IS are thus restored to normal.

What is claimed is:

1. In a telephone system, an outgoing trunk circuit. an incoming trunk circuit. means comprising a two-conductor trunk interconnecting said circuits. a loop signaling circuit including in series a battery one pole of which is connected to ground, both windings of a line relay and both windings of a differential relay in said incoming trunk circuit, the conductors of said trunk and the windings of marginal and polarized relays in said outgoing trunk circuit, said marginal and line relays only being operatively energized in said loop circuit, means in said incomingr trunk circuit for reversingT the current in said loop signaling circuit to eiect the operation of said polarized relay. means in said outgoing trunk circuit for opening the loop signaling circuit and connecting ground potential to one conductor of said trunk to effect the operation of said differential relay and to hold operated said line relay in said incoming trunk circuit, and means in said incoming trunk circuit for decreasing the voltage applied through one winding of the line relay and one winding of the differential relay to effect the release of the marginal relay in said outgoing trunk circuit Without releasing the polarized relay therein.

2. In a telephone system, an outgoing trunk circuit, an incoming trunk circuit, means including a two-conductor trunk for interconnecting said trunk circuits, a marginal relay and a polarized relay in said outgoing trunk circuit, a line relay and a differential relay in said incoming trunk circuit, each of said relays having two windings, a battery one pole of which is connected to ground, means bridging said battery and both windings of said line and differential relays across the conductors of said trunk, means in said outgoing trunk for connecting the windings of said marginal and polarized relays in series across the conductors of said trunk thereby to effect the operation of only the marginal relay in said outgoing trunk circuit and only the line relay in said incoming trunk circuit, means in said incomng trunk circuit for interchanging the the connections between the windings of the line relay and the windings of the differential relay to reverse the current through the conductors of said trunk and thereby effect the operation of the polarized relay in said outgoing trunk circuit, means in said outgoing trunk circuit for opening the series connection through said marginal and polarized relays and for connecting ground potential through the winding of said marginal relay to a single conductor of said trunk to effect the operation of the differential relay in said incoming trunk circuit without releasing said line relay, and means in said incoming trunk circuit for decreasing the current through the conductors of said trunk and the windings of said marginal and polarized relays in said incoming trunk circuit to cause the release of said marginal relay.

3. In a telephone system, an outgoing trunk circuit, an incoming trunk circuit, means including a two-conductor trunk for interconnecting said trunk circuits, a marginal relay and a polarized relay in said outgoing trunk circuit, a line relay and a differential relay in said incoming trunk circuit, each of said relays having two indings, a battery one pole of which is connected to ground, means bridging said battery and both windings of said line and differential relays across the conductors of said trunk, means in said outgoing trunk for connecting the windings of said marginal and polarized relays in series across the conductors of said trunk thereby to effect the operation of only the marginal relay in said outgoing trunk circuit and only the line relay in said incoming trunk circuit, means in said incoming trunk circuit for interchanging the connections between the windings of the line relay and the windings of the diierential relay to reverse the current through the conductors of said trunk and thereby eiect the operation of the polarized relay in said outgoing trunk circuit, means in said outgoing trunk circuit for opening the series connection through said marginal and polarized relays and for connecting ground potential through the winding of said marginal relay to a single conductor of said trunk to eiect the operation of the differential relay in said incoming trunk circuit without releasing said line relay, and means in said incoming trunk circuit for shunting the loop through the conductors of said trunk and the windings of the marginal and polarized relays in said incoming trunk circuit thereby to cause the release of the marginal relay without releasing the polarized relay.

LINDLEY A. ICILLE. THEODORE D. ROBB.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,990,437 Moody Feb. 5, 1935 2,298,751 Crocker Oct. 13, 1942 

